C. BRENT BARGERON, RICHARD C. BENSON, ROBB W. NEWMAN, A. NORMAN JETTE, and TERRY E. PHILLIPS

OXIDATION MECHANISMS OF HAFNIUM CARBIDE AND HAFNIUM DIBORIDE IN THE TEMPERATURE RANGE 1400 TO 2100°C

Two ultra-high-temperature materials, hafnium carbide and hafnium diboride, were oxidized in the temperature range 1400 to 2100°C. The two materials oxidized in distinctly different ways. The carbide formed a three-layer system consisting of a layer of residual carbide, a layer of reduced (partially oxidized) hafnium oxide containing carbon, and a layer of fully oxidized hafnium dioxide. The diboride oxidized into only two layers. For the diboride system, the outer layer, mainly hafnium dioxide, contained several intriguing physical structures.

INTRODUCTION Materials that can provide protection at temperatures aspects of research that has been performed over the past above l700°C in an oxidative environment are needed for four or five years.2-5 important applications. To be usefully employed as a turbine blade coating, for example, a substance would EXPERIMENTAL METHODS need to withstand many excursions from normal ambient The experimental arrangement for the oxidation pro­ conditions into the high-temperature regime and back cess has been described in detail previously.2-4 An induc­ again without cracking, spalling, or ablating. Other ap­ tion furnace consisting of two concentric zirconia tubes plications, such as a combustion chamber liner, might with a graphite susceptor between them was used to heat require only one high-temperature exposure. Not only do the specimen. (A susceptor is the heating element in an the chemical properties need to be considered, but the induction furnace.) The specimen temperature was mea­ physical, microscopic structure of a candidate material sured with an optical pyrometer through a sapphire win­ can also determine how well it will function under ex­ dow. Gas flow through the furnace of about 20 cm/s was treme conditions. To illustrate, a substance fabricated by controlled with a system of solenoid valves that main­ hot-pressing a powder will generally have micrometer­ tained a flow of argon until the desired temperature was size pores throughout, which allow hot oxidative gases reached, at which time oxygen, at a partial pressure of 55 to diffuse at a significantly higher rate than in a material torr, was added to the furnace atmosphere. After a spec­ of the same chemical composition made by chemical ified oxidation time, the atmosphere was switched back vapor deposition (CYD). Carefully manufactured CYD to pure argon, the furnace was turned off, and the spec­ films do not have pores or cracks, so hot gases must imen was allowed to cool to room temperature. diffuse through the lattice of the bulk material, a much The specimen was then removed from the zirconia slower process than diffusion through pores or openings. tube, photographed at several magnifications, embedded The oxidation mechanisms of CYD films of two impor­ in epoxy, cut to reveal a cross section of the oxidized film, tant high-temperature materials are discussed in this and finally polished using a series of successively finer article. Hafnium carbide and hafnium diboride melt at diamond grit. Normally, samples were sectioned by cut­ 3950 and 3250°C, respectively. In an oxygen atmosphere, ting perpendicular to the surface. Occasionally, however, both form hafnium dioxide, which melts at about a specimen was cut at a very small angle to the surface 2900°C. In addition to at least one solid oxide, both to expose broader sections of oxide and residual material substances produce other oxidation products. Hafnium for X-ray diffraction measurements. Thickness measure­ carbide yields either CO or CO2, and hafnium diboride ments of the various layers were made with a calibrated produces boric oxide, which becomes a liquid at 450°C Olympus metallurgical microscope, and cross-sectional and vaporizes at 1860°C. Obviously the presence of photographs were taken. The polished cross sections were gases and liquids in a protective film can have significant also examined by X-ray microanalysis in a scanning ramifications. Further, a structural phase transition that electron microscope (SEM). occurs in hafnium dioxide at about 1700°C should not To characterize certain layers, electrical resistance was be overlooked. This transition entails an accompanying measured as a function of temperature. Leads were at­ 3.4% volume changel that can result in cracking or spall­ tached successively to the polished surfaces of the indi­ ing. The work presented here is a selection of various vidual layers, and the specimen was then placed in a f ohns Hopkins APL Technical Digest, Vo lume 14, Number 1 (/993) 29 C. B. Bargeron et ai.

cryostat, where the resistance was measured as the tem­ observe hollow channels in the protrusions. Some of the perature was lowered to near 10K and then returned to stovepipes have a height almost equal to the film thick­ room temperature. The hardness numbers were deter­ ness. The formation of these structures is apparently an mined for the oxidized hafnium carbide film using stan­ intricate process that includes the transport and deposi­ dard procedures employing a Knoop indenter. Polished tion of hafnium dioxide by liquid or gaseous B20 3. Figure surfaces were also employed for the latter measurements. 4 is a stereopair of photographs of one of the stovepipes, revealing in dramatic fashion not only the hollow center RESULTS of the protrusion but also a large channel going into the A photograph of the hafnium diboride film before ox­ oxidized layer at the left of the structure's base. idation is shown in Figure l. The polycrystalline film has Two examples of oxide growth morphology formed at well-formed crystallites with flat faces and well-defined 1900°C are shown in Figure 5. The hafnium diboride edges. The overall integrity of the coating is good; only exposed for 100 s (Fig. 5A) has an interesting growth an occasional crack is present. On the basis of the exam­ pattern in which the oxide has periodically separated into inations of cross sections revealing the film/substrate in­ thin laminae. The film exposed to oxygen for 300 s (Fig. terface, the attachment of the coating to the substrate is 5B) provides an example of the formation of large voids. believed to be excellent. Similar observations were made In both examples, the separation of oxide and the remain­ for the hafnium carbide films. ing diboride is apparent. Figure 2 shows a cross section of the film after expo­ Examination of cross sections of hafnium carbide re­ sure to oxygen for 1800 s at 1520°C. At this temperature, vealed a phenomenon different from that of hafnium a relatively compact oxide forms. Cracks in the oxide diboride in that an additional layer had formed between between many of the grains were probably formed during the residual carbide and the white outer layer of oxide. cooling or post-oxidation handling. Oxide formed at this In the light microscope, the new layer appeared gray (Fig. temperature appears to adhere to the residual diboride. At 6). In the SEM, the interlayer was observed to be compact higher oxidation temperatures, however, separation be­ and fine grained compared with the porous outer oxygen tween the oxide and diboride is common and is probably layer shown in Figure 7. This new layer was very intrigu­ caused by different temperature coefficients of expansion ing from the point of view of protecting the substrate, for the oxide and diboride. since compact layers are expected to allow much slower Near the boiling point of boric oxide C::d860°C), the transport of gases than porous materials. To characterize oxidized hafnium diboride develops some interesting the interlayer, further analysis was performed. morphology. Figure 3 is a stereopair of photographs of Figures 8A, 8B , and 8C show X-ray microanalysis the oxide after exposure to oxygen for 540 s at 1850°C. spectra of the residual carbide, the interlayer, and the Many blunt posts protrude from the surface, a few of outer oxide layer, respectively. As noted earlier,3 the big which have no cover over the top, suggesting that they change in the oxygen-to-hafnium peak ratio occurs be­ are hollow, resembling stovepipes. In stereo, one may tween the residual carbide and the interlayer. In other

Epoxy

Oxide

Diboride

20 ttm Figure 2. Cross section of film after exposure to 55 torr of oxygen for 1800 s at 1520°C. The oxide is compact but has some cracks I 40 ttm I between the grains. The cracks probably formed during cooling or Figure 1. Surface of hafnium diboride film showing crystallites post-experimental handling. Oxide formed at this temperature with flat faces and well-defined edges. appears to adhere to the diboride in most places.

30 Johns Hopkins APL Technical Digest, Vo lume 14, Number 1 (1 993) Oxidation Mechanisms of Hafnium Carbide and Hafnium Diboride

Epoxy Figure 3. A stereopair of photographs of the cross section of a film oxidized in 55 torr of oxygen for 540 s at 1850°C, showing unusual protrusions from the f· oxide surface, Viewing in stereo reveals 'c·:-: , .. ,.s'O,C' " Oxide that these structures are hollow (see also Fig . 4), resembling stovepipes, with a well-formed central channel. In fact, many channels occur throughout the Gap films oxidized at this temperature, sev­ eral of which can be observed in this stereopair. Note also that the oxide is Diboride separated from the residual diboride. == Substrate 200 JLm

Figure 4. A higher-magnification stereopair of photographs of the stove­ pipe on the left in Figure 3. Viewing in stereo reveals a hollow, smooth-walled central tube leading out of the oxide. The cracking in the wall is probably due to post-experimental handling. Nearthe base of this protrusion on the left side, a large secondary channel exists in which one can see well-formed crystallites on the wall.

50 JL m

words, the oxygen-to-hafnium peak ratio is much the The results of the electrical resistance determinations same in the interlayer and the outer oxide layer. The peak are shown for the interlayer in Figure 9 and for the ratio of carbon to hafnium changes most in the interlayer/ residual carbide in Figure 10. The outer oxide had high outer oxide interfacial region. To summarize, the mi­ electrical resistance; no results are shown for that layer. croanalysis results indicate that the interlayer is an oxide The residual carbide layer and the interlayer have rather with carbon impurity rather than a carbide. metallic-like resistance values above 50 K. Below 50 K, X-ray diffraction analysis indicated that the hafnium however, the resistance of both layers increases, indicat­ carbide had the usual cubic structure with a lattice con­ ing that both materials more probably fall in the category stant of 4.60 ± 0.01 A. As shown in Table 1, the outer of degenerate semiconductors. The upswing in resistance oxide layer had the known monoclinic structure. It was is especially apparent for the in terlayer. surprising, considering the sharply delineated interfaces Knoop hardness numbers were 970 and 986 kg/mm2 of the oxidized film (Figs. 6 and 7), that the interlayer for the residual carbide and interlayer, respectively, 2 structure (also in Table 1) matched that of the outer oxide whereas the outer layer had a value of 297 kg/mm , or layer as closely as it did. Why should this be so? If the about one-third of the hardness of the other two. This interlayer and the outer oxide have the same crystal struc­ ratio is consistent with handbook hardness numbers for ture, one would expect the amount of oxygen to change hafnium dioxide and hafnium carbide. gradually from the carbide/interlayer interface through the interlayer into the outer oxide, with no visual interface DISCUSSION between the two layers. It seems likely, therefore, that Exploiting information obtained experimentally, we although the crystal structures are similar at room temper­ can deduce the oxidation mechanisms of hafnium carbide ature, they are not the same at the elevated temperature; and hafnium diboride. It seems clear that hafnium carbide otherwise, the observed sharp boundary between the two dissolves oxygen into its lattice. From our data, Figure materials would not be expected. 8A shows a small oxygen X-ray peak for the residual

Johns Hopkins APL Technical Digest, I/oilime 14, Number I (1993) 31 C. B . Bargeron et af.

A

Epoxy

==~ Oxide Hf0 _ C 2 X y Gap ~ Oxide Gap -~Oxide HfC == Gap Substrate

Diboride Figure 6. Light micrograph of a cross section of oxidized hafnium carbide film showing a multilayer structure with well-defined inter­ faces. The adhesion between all layers appears to be excellent with no signs of spalling or separation . Oxidation was for 600 s at = Substrate 1865°C in an atmosphere of 93% argon and 7% oxygen. 100 J.Lm B

Epoxy

Oxide

==Gap

Diboride

Substrate

I 200 J.Lm

Figure 5. Two examples of oxides grown in 55 torr of oxygen at 1900°C. A. A film exposed for 1 00 s, where the oxide has separated into thin laminae. B. A film exposed for 300 s, which has large voids Figure 7. Scanning electron micrograph of a cross section of in several places. Apparently, at this temperature, conditions are hafnium carbide film oxidized at 1865°C for 600 s in 7% oxygen and argon. The compactness and lower porosity of the Hf0 _ no longer favorable for the stovepipe growths in Figures 3 and 4. 93% 2 XCy Again, the oxide and residual diboride have separated. (i nterlayer) are apparent.

Table 1. X-ray diffraction parameters.

X-ray

diffraction Hf02 Hf0 2 Hf02 White Gray parameter (Ref. 6) (Ref. 7) (Ref. 8) outer oxide interlayer a (A) 5.1156(5) 5.119 5.12 5.10(1) 5.14(1) b (A) 5.1722(5) 5.169 5.18 5.16(1) 5.19(1) c (A) 5.2948(5) 5.290 5.25 5.27(1) 5.27(1) (3 (deg) 99.18(8) 99.25 98 99.1 (1) 99.2(1) V (A3) 138.20 138.15 137.9 136.9 138.8 Notes: All X-ray diffraction results indicate monoclinic structures. The values a, b, and c are unit cell dimensions; (3 is the angle between edges a and c; and V is the unit cell volume. Our results for the outer oxide and interlayer were determined by a weighted fit of23 and 28 diffraction lines, respectively, obtained with a Read camera using filtered Cr radiation. Polycrystalline aluminum foi l placed on the surface of the target oxides was employed as a reference to calibrate the camera. The aluminum, which had a slit in it, also served as a mask so that data were collected from only one layer at a time. The numbers in parenthe es are the uncertainties in the least significant digits.

32 Johns Hopkins APL Technical Digest, Volume 14, Number 1 (1993) Oxidation Mechanisms of Hafnium Carbide and Hafnium Diboride

A 20 0.009 ,------,------,------, Hf

16 (j) (j) "0 ~ 0.008 Cro .s OJ ~12 () c 0 ro -£ U) c ·00 0.007 ::::.- OJ C/) 8 a: C ::J 0 () 4 0.006 :------,--JOL - ----,--!0-=-0------,,-J300 Hf 0 1 O 2 Temperature (K) 0 Figure 9. Resistance versus temperature for the reduced hafnium B 20 Hf oxide interlayer. The upswing in resistance below 50 K suggests that this new material is a degenerate semiconductor. The units of the ordinate are sheet resistance (ohms). Sheet resistance is the _16 C/) electrical resistance of a material with the dimensions of a unit "0 Cro square and unspecified thickness. C/) 6 12 -£ .~

C/) C 8 ::J 0 () 0 0.008 ,------,------,------, 4 0.007 (j) E 0 .l:: .s 0.006 C 20 OJ () c ro 0.005 Hf .~ C/) 16 OJ (j) a: "0 0.004 Cro ~12 0 0.003 L-_____...L- _____ ...l...-_____---.J -£ o 100 200 300 .~ Temperature (K) C/) 8 C Figure 10. Resistance versus temperature forthe residual hafnium ::J 0 carbide layer. The slight increase in resistance below 50 K is () 0 4 possibly suggestive of a degenerate semiconductor. The units of the ordinate are sheet resistance (ohms). Sheet resistance is the electrical resistance of a material with the dimensions of a unit 0 square and unspecified thickness. 0 2 3 4 5 Energy (keV) Figure 8. A. X-ray spectrum of the residual hafnium carbide layer, indicating the presence of oxygen. The gold peak is a result of a in this manner. Therefore, as the carbide encounters coating applied to the specimen to prevent charging in the scanning oxygen, it fIrst absorbs it into the bulk lattice. electron microscope. The hafnium carbide film was oxidized at As the oxygen concentration increases in the carbide, conditions described in the Figure 6 caption. B. X-ray spectrum of it reaches a saturation level, as discussed in the preceding the interlayer. C. X-ray spectrum of the outer oxide layer. paragraph. Even though a percentage of the carbon has probably left the carbide lattice, considerable carbon still is present in the carbide lattice when saturation is at­ hafnium carbide. Further, in an extensive study of mate­ tained. Thus, as the new material forms , a considerable rials made from various mixtures of hafnium carbide, amount of carbon is entrained in it. From our microan­ hafnium dioxide, and hafnium nitride, Constant et al. 9 alytical measurements, the new material has the form of showed that the hafnium carbide can maintain a single­ Hf02- xCy , where x is small compared with 2 and y is less phase, cubic lattice with as much as 25% of the atomic than the formula value that carbon possessed at the last carbon being replaced by atomic oxygen at l600°C, and instant in the carbide. The carbon diffuses outward as that the percentage increases to 30% at 2000°C. In other oxygen continues to move inward. Eventually, the carbon words, one can synthesize a composition of HfC1-xOx level is low enough and the oxygen level high enough that where x

Johns Hopkins APL Technical Digest. Volume 14. Number 1 (1993) 33 C. B. Bargeron et al.

3 layer possesses the tetragonal structure. Either the carbon we employ theoretical densities of PI == 10.1 g/cm and 3 or the reduced amount of oxygen, or both, in the inter­ P2 = 12.7 g/cm in the equation f2P2[Xi(t)-X2(0)] layer must prevent the interlayer and the outer oxide from = fIPI[xt(t) -Xl(O)], from which we solve for having the same structure at elevated temperatures. If this [ xt (t) - Xl (0)]. In the preceding equation, 11 is the

were not so, no reason would exist for the interlayer/ weight fraction of Hf in Hf02, and 12 is the weight fraction oxide interface to form as experimentally observed. of Hf in HfC; thus, the equation conserves Hf atoms. Thus, as hafnium carbide oxidizes, three distinct layers Implicit in our scheme is the growth of the outer oxide form in a dynamic system with moving interfaces and at xt (t), the depletion of the interlayer oxide at Xl (t), with each layer possessing a unique density (inferred the growth of the interlayer oxide at xt (t), and the from Fig. 7 in conjunction with other evidence). Several depletion of the carbide at X2(t). We neglect all compli­ years ago, Danckwerts to solved the diffusion equation for cations due to the outward (i.e., in the negative direction a two-layer system with a moving boundary.ll Our in Fig. 11) diffusion of carbon. We consider only the model, depicted in Figure 11, extends his treatment to inward diffusion of oxygen and recognize that at our include a third layer.4 Locations in each of the three layers working temperatures, 20 to 30% of the carbon sites in are given by Xo, Xl> and X 2> respectively. Each of the three the hafnium carbide lattice must be occupied by oxygen coordinate systems is fixed in its particular medium as before the structure transforms to an oxide.9 shown. Thus, during oxidation, the systems are in relative Because coordinate systems are not moving relative to motion to one another. Interfaces are denoted by Xlt) , their own medium, the diffusion equations can be written where i = 0, 1, 2 refers to coordinate systems and t rep­ simply as resents time. In general, two interfaces are associated with each layer; these are distinguished by superscript + and - signs. The initial position of an interface is Xi(O). where Ci(Xi, t) is the concentration of the diffusate in the The indices i = 0, 1, and 2 refer to the outer oxide, the interlayer oxide, and the residual carbide, respectively. ith medium at point Xi and D i is the diffusion constant. At each interface, the diffusate is conserved according to The positions XI(O) and X 2(0) are indicated by dotted lines because they are not seen in the cross section after oxidation. We know where XiO) is located by measure­ DJJCi(xj, t)/ dXjk=\ + Ci(Xi , t)dXj / dt ment before oxidation. The total thickness of interlayer xt (t) - = Dj +l dCi +l (xi+l,t) / dXi+ll x- I=X, formed, Xl (0), is a quantity necessary to solve l+ z+l (2) the overall diffusion system and is estimated by (1) noting that all of our photographs suggest that the interlayer +Ci+1(Xi +1, t)dXi+l / dt oxide is dense (i.e., without voids or pores); (2) knowing for i = 0 and 1. The first term on either side of the equation from X-ray microanalysis and diffraction that it is an represents the diffusate flux across the interface, whereas oxide-type medium; and (3) obtaining, by subtraction, the the second term takes into account growth and depletion, amount of carbide depleted. Thus, (1) and (2) suggest that resulting in moving boundaries. The solutions of Equation 1 are well-known error functions that depend on the diffusion constants D i, I I Residual parabolic rate constants obtained from experiment, and I I carbide concentrations based on the results of the X-ray mi­ Initial I I croanalysis. The diffusion constants are determined from carbide I I the boundary conditions given by Equation 2. The inter­ Interlayer ested reader will fmd further details in Ref. 4. The dif­ I I , I I fusion constants obtained in this manner are given in Xo X1 Table 2. ~ ~ ~ Total interlayer I I The diffusion constants in the table indicate that the formed interlayer oxide is a diffusion barrier for oxygen under I I Monoclinic our experimental conditions. With regard to the diffusion I I oxide constants for the outer oxide, we know from thickness I I measurements that this layer is much thicker (and there­ I I fore less dense) than it would be if the oxide possessed X2(O) X; (t) its theoretical density. The lower density probably con­ tributed to a larger diffusion constant in this layer, and X1-(t) our results for the outer oxide should not be applied to Figure 11. A schematic diagram of the hafnium carbide oxidation fully densified hafnium dioxide. At the higher tempera­ model. Lowercase XO' Xl ' and x2 indicate coordinate systems that are fixed in each of the respective media. During oxidation, these ture, the outer oxide layer should possess the tetragonal systems move with respect to one another because of volume structure during the oxidation. changes due to structural transformation taking place at the inter­ The interlayer oxide is composed of only three ele­ faces. Uppercase X's indicate the positions of various interfaces in ments (Hf, C, and 0). From our micrographs, this new the cross section of the oxidized film. Solid interfacial lines are observable in the cross section. Dashed lines represent original material appears to adhere extremely well to both the interfaces that no longer exist because of layer depletion. See the residual carbide and the outer oxide. Specifically, we text for further details of this model. have seen no cracking, spalling, or separation at any of

34 f ohns Hopkins APL Technical Digest, Volume 14, Number 1 (1993) Oxidation Mechanisms of Hafnium Carbide and Hafnium Diboride

Table 2. Oxygen diffusion constants for different layers nace. In qualitative chemical analysis, the white powder at various temperatures. tests positively for boron. Boric oxide melts at about 450°C, so at 1675°C it is present in the oxide as a liquid, Temperature Diffusion constant in which form it probably acts to seal any porosity or Layer (OC) (cm2/s) cracks and contributes to the slower oxidation rate. Outer oxide 1400 8.1 X 10 8 Therefore, in the oxidation of hafnium diboride, no 2060 3.0 X 10- 6 oxygen is absorbed into the bulk, culminating in a phase change when a saturation level is reached. Instead, a 9 Interlayer 1400 7.9 X 10- chemical reaction takes place directly at the interface, 7 oxide 2060 1.1 X 10- releasing gaseous products that form large pathways Carbide 1400 2.6 X 10- 7 through the oxide. This result allows additional oxygen 2060 1.6 X 10- 5 to reach the interface more expeditiously, thus beginning the cycle again. Our work has shown that hafnium carbide and hafnium its interfaces with either of the other two layers. In ad­ diboride films oxidize in quite different ways at elevated dition, we have not observed cracks or voids within the temperatures. The carbide forms a fine-grained, compact material itself. Our observations of this new material protective interlayer that slows the diffusion of oxygen. suggest that it might be a very useful and protective high­ In contrast, the diboride forms gaseous products at the temperature substance. Currently, no reasons are known interface, creating voids and easy oxygen access. The why the material could not be produced as a monolithic formation of the interlayer during the hafnium carbide protective film. oxidation also has the apparent benefit of matching It is instructive to compare our results with those of materials together better, probably with respect to both others who have investigated the oxidation of hafnium interfacial chemical adhesion and coefficients of thermal carbide. Only two groups of investigators will be men­ expansion, such that they do not separate from one an­ tioned here. Berkowitz-Mattuckl2 used arc-melting tech­ other during broad temperature excursions. niques to make samples. She observed grain boundary oxidation in her specimen, which often fell apart during oxidation at temperatures up to 1730°C, which was her maximum value. In this instance, it seems that grain REFERENCES boundary impurities, probably carbon, were oxidizing I Lynch, C. T. , in High Temperature Oxides, Part II, Alper, A. M. (ed. ), much faster than the bulk. Recently, Prater et al. 13 pub­ Academic Press, New York (1970). 2Bargeron, C. B. , and Benson, R. c., High Temperature Oxidation of Hafnium lished their research on hot-pressed powders of hafnium Carbide, NASA CP-3054, Part 1, pp. 69-82 (1989). carbide. In their paper, curiously, they do not mention the 3Bargeron, C. B. , and Benson, R. c., "X-ray Microanalysis of Hafnium formation of an interlayer at all. In the Ph.D. dissertation Carbide Films Oxidized at High Temperature," SUI! Coat. Tecl7l1 01. 36, 111- 115 (1988). of Holcomb, 14 however, who is one of the authors of Ref. 4Bargeron, C. B., Benson, R. c., and Jette, A. N., High-Temperature Diffusion 13, the interlayer is mentioned rather often, and specu­ of Oxygen in Oxidizing Hafnium Carbide Films, NASA CP-3054, Part 1, pp. 83-94 (1989). lation is presented about its possible nature. It is likely 5Bargeron, C. B. , Benson, R. C., Jette, A. N., Newman, R. W., and Paquette, that the interlayer was not as prominent in the specimens E. L. , Oxidation MOIphology and Kinetics of Hafnium Diboride at High of Prater et al.13 because the hot-pressed samples had Temperature, NASA CP-3097, Part 2, pp. 545-554 (1990). 6 Adam, J., and Rogers, M. D., "The Crystal Structure of Zr02 and Hf02," Acta micrometer-size pores that allowed greater oxygen diffu­ Crystallogr. 12, 951 (1959). sion to the critical region and, thus, oxidized the inter­ 7Ruh, R. , Garrett, H. J. , Domagala, R. F., and Tallan, N. M., "The System layer at a greater rate, keeping it thin. Zirconia-Hafnia," 1. Am. Ceram. Soc. 51, 23-27 (1968). 8 Geller, S. , and Corenzwit, E. , "Crystallographic Data: Hafnium Oxide, Hf02 In regard to the oxidation of hafnium diboride, no (Monoclinic)," Anal. Chem. 25, 1774 (1953). evidence seems to indicate that hafnium diboride dis­ 9Constant, K., Kieffer, R., and Ettmayer, P. , "On the Pseudotemary System ' HfO'-HfN-HfC," Monatsh. Chem. 106, 973-981 (1975). solves oxygen into its bulk. In addition, near and above IODanckwerts, P. V. , "Unsteady-State Diffusion or Heat Conduction with the boiling point of boric oxide, large voids and stove­ Moving Boundary," Trans. Faraday Soc. 46, 701-712 (1950). pipes form in the oxide layer, indicating a prominent I I Crank, J., The Mathematics of Diffusion, Clarendon Press, Oxford, England (1975). gaseous presence in the film, even though the oxygen was 12Berkowitz-Mattuck, J. B. , "High-Temperature Oxidation: IV. Zirconium and maintained in the initial gas stream at the same level as Hafnium Carbides," 1. Electrochem. Soc. 114, 1030-1033 (1967). during the oxidation of hafnium carbide. The large voids 13Prater, 1. T. , Courtright, E. L. , Holcomb, G. R. , St. Pierre, G. R., and Rapp, R. A. , Oxidation of Hafnium Carbide and HfC with Additions of Tantalum and other paths through the oxide mean easier access to and Praseodymium, NASA CP-3097, Part I, pp. 197-209 (1990). the interface where the oxidation takes place, creating 14Holcomb, G. R., The High Temperature Oxidation of Hafnium Carbide, Ph.D. additional gaseous products that then must be disposed dissertation, The Ohio State University, Columbus, Ohio (1988). of. Evidence indicates that B20 3 is released from the oxidizing film. After the higher-temperature (3) 1850°C) hafnium diboride tests, one finds a water-soluble, white ACKNOWLEDGMENT: Dennis Wilson of the Technical Services Department powder deposited downstream on the walls of the fur- cut and polished the specimens and performed the hardness measurements.

Johns Hopkins APL Technical Digest, Volume 14, Number I (1993) 35 C. B. Bargeron et al.

THE AUTHORS

C. BRENT BARGERON earned a ROBB W. NEWMAN received Ph.D. degree in physics at the his B.S. degree in mechanical en­ University of Illinois in 1971 and gineering from Cornell University joined APL that year as a member in 1965 and his master's degrees of the Milton S. Eisenhower Re­ in space technology, administra­ search Center. Since joining APL, tive science, and computer science Dr. Bargeron has been involved in from The Johns Hopkins Univer­ problems in solid state physics, sity. He joined APL in 1966 and light scattering, chemical lasers, has been involved in programs in arterial geometry, corneal damage transpiration cooling, laser heat­ from infrared radiation, mineral ing, and high-temperature materi­ deposits in pathological tissues, als testing. He has served as co­ quality control and failure analysis chair of the Standard Missile, BLK of microelectronic components, IV Airframe Structure Coordinat­ electron physics, and surface sci­ ing Committee. Mr. Newman is ence. currently interested in intelligent systems and is supervisor of the Applied Intelligent Systems Section of the BumbleBee Engineering Group.

RICHARD C. BENSON received A. ORMAN JETTE received his a B.S. in physical chemistry from Ph.D. in physics from the Univer­ Michigan State Unjversity in 1966 sity of California, Riverside, in and a Ph.D. in physical chemistry 1965. Dr. Jette joined APL that from the University of Illinois in year and has worked in the Milton 1972. Since joining APL in 1972, S. Eisenhower Research Center on he has been a member of the theoretical problems in atomic, Milton S. Eisenhower Research molecular, and solid-state physics. Center and is currently supervisor In 1972 he was visiting professor of the Matelials Science Group. of physics at the Catholic Univer­ He is involved in research on the sity of Rio de Janeiro, Brazil, and properties of matelials used in in 1980 he was visiting scientist at microelectronics and spacecraft, the Center for Interdisciplinary and the application of optical tech­ Research at the University of niques to surface science. Dr. Bielefeld, Federal Republic of Benson has also conducted re­ Germany. search in Raman scattering, opti­ cal switching, laser-induced chemistry, chemical lasers, energy trans­ fer, chemiluminescence, fluorescence, and microwave spectroscopy. He is a member of l£EE, the American Physical Society, the American Vacuum Society, and the Materials Research Society.

TERRY E. PHll.,LIPS received a B.A. from Susquehanna Univer­ sity and Ph.D. in organic chemis­ try from The Johns Hopkins Uni­ versity in 1976. After completing postgraduate studies at Northwest­ ern University in low-dimensional organic conductors, he joined APL in 1979, where he is a chemist in the Materials Science Group of the Milton S. Eisenhower Re­ search Center. He has studied photoelectrochemical energy con­ version; inorganic optical and electrical phase transition com­ pounds; hjgh-temperature super­ conductors; and material charac­ terization with X-rays, nuclear magnetic resonance, mass spectros­ copy, and optical spectroscopic techniques. He is a member of the American Chemical Society, the American Physical Society, and the Materials Research Society.

36 Johns Hopkins APL Technical Digest, Volume 14, Number I (1993)